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Home , Arapahoe Formation, Cryogenian, Dawson Arkose, Great Unconformity, Pikes Peak granite, Sawatch Formation

Diachronous development of Great before Snowball

Rebecca M. Flowersa,1, Francis A. Macdonaldb, Christine S. Siddowayc, and Rachel Havraneka

aDepartment of Geological Sciences, University of , Boulder, CO 80309; bEarth Science Department, University of California, Santa Barbara, CA 93106; and cDepartment of Geology, The Colorado College, Colorado Springs, CO 80903

Edited by Paul F. Hoffman, University of Victoria, Victoria, Canada, and approved March 6, 2020 (received for review July 30, 2019) The Great marks a major gap in the continental precisely because the Great Unconformities mark a large gap in geological record, separating basement from Phan- the rock record, the history leading to their formation erozoic sedimentary rocks. However, the timing, magnitude, cannot be investigated directly by study of preserved units. spatial heterogeneity, and causes of the erosional event(s) and/ Past work leads to at least four general models for the timing or depositional hiatus that lead to its development are unknown. and magnitude of pre-Great Unconformity continental erosion, We present field relationships from the 1.07-Ga batho- which are depicted in Fig. 1. Some have proposed major erosion lith in Colorado that constrain the position of Cryogenian and of the continents associated with assembly of the supercontinent paleosurfaces below the Great Unconformity. Tavakaiv and mantle upwelling below it prior to 850 Ma (Hy- injectites with an age of ≥676 ± 26 Ma cut Pikes Peak pothesis 1) or with the early diachronous breakup of Rodinia . Injection of quartzose sediment in bulbous bodies indi- between 850 and 717 Ma (Hypothesis 2) (10–17). Others argue cates near-surface conditions during emplacement. Fractured, for an association with the Cryogenian glacia- weathered wall rock around Tavakaiv bodies and intensely altered tions from 717 to 635 Ma (Hypothesis 3) (18–22). Still others basement fragments within unweathered injectites imply still ear- suggest that the appearance of and modern ecosystems lier regolith development. These observations provide evidence was the product of nutrient delivery via erosion and an increase that the granite was exhumed and resided at the surface prior in atmospheric oxygen at the –Cambrian transition at to sand injection, likely before the 717-Ma for ca. 540 Ma (Hypothesis 4) related to the “transgressive buzzsaw” the climate appropriate for regolith formation over an extensive and subsequent burial (4, 5), later rifting of Laurentian margins region of the paleolandscape. The 510-Ma Sawatch sandstone di- (23, 24), or the Pan-African and Transantarctic and rectly overlies Tavakaiv-injected Pikes granite and drapes over construction of the supercontinent (25, 26). core stones in Pikes regolith, consistent with limited erosion be- A critical question is if the Great Unconformity represents a tween 717 and 510 Ma. Zircon (U-Th)/He dates for basement be- single, global exhumation pulse (2, 4, 5) as the models above low the Great Unconformity are 975 to 46 Ma and are consistent typically assume, or if it is composite in origin (27). It is possible with exhumation by 717 Ma. Our results provide evidence that most that there are multiple Great Unconformities, which formed erosion below the Great Unconformity in Colorado occurred before over hundreds of millions of due to diachronous regional the first Neoproterozoic Snowball Earth and therefore cannot be a tectonic phenomena. Multiple spatially heterogeneous exhuma- product of glacial erosion. We propose that multiple Great Uncon- tion events driven by several of the above mechanisms could formities developed diachronously and represent regional tectonic combine to generate one amalgamated gap of lengthy duration features rather than a synchronous global phenomenon. over a broad geographic scale. Arguments that Great Un- conformity erosion surface formation was a global phenomenon Great Unconformity | Snowball Earth | thermochronology | zircon (U-Th)/ use geochemical proxy data or preserved sediment volumes on He | injectites North America to indirectly infer erosion flux and nutrient de- livery across the Great Unconformity (2, 4, 5) rather than di- he Great Unconformity is an iconic geologic feature that rectly analyzing the age of the erosion surfaces. However, Tclassically marks the boundary between unfossiliferous Pre- interpretations of these geochemical records are nonunique, and cambrian rocks and fossiliferous strata. To Charles the surfaces themselves are poorly dated. Deciphering the age of Darwin (1), the “sudden appearance” of complex macroscopic in Cambrian strata necessitated a global stratigraphic Significance omission to reconcile the record with Darwinian gradual- ism. Although subsequent concepts of extinction and radiation Erosion below the Great Unconformity has been interpreted as eliminated the need of a global time gap to explain the fossil a global phenomenon associated with Snowball Earth. Geo- record, the notion of a Late Neoproterozoic to Cambrian global logical relationships and thermochronologic data provide evi- unconformity has persisted (2–5). Particularly, the Great Un- dence that the bulk of erosion below the Great Unconformity conformity in North America, where the base of the Cambrian in Colorado occurred prior to Cryogenian glaciation. We sug- transgressive sequence commonly overlies Precambrian base- gest that there are multiple, regionally diachronous Great Un- ment (6), has been globally correlated with Late Neoproterozoic conformities that are tectonic in origin. to Cambrian unconformities on other continents (2–5). Recently, inferred erosion across these Great Unconformities has been Author contributions: R.M.F. and F.A.M. designed research; R.M.F., F.A.M., C.S.S., and R.H. associated with a variety of changes in the Earth , in- performed research; and R.M.F., F.A.M., and C.S.S. wrote the paper. cluding the Neoproterozoic Snowball Earth (2), the initiation of Competing interest statement: P.F.H. and F.A.M. are coauthors on three papers, most modern plate tectonics (3), oxygenation of the ocean and at- recently in 2017. mosphere (4), and the (5). However, the This article is a PNAS Direct Submission. timing, magnitude, and causes of erosion below the Great Un- Published under the PNAS license. conformities, and whether their development was globally syn- 1To whom correspondence may be addressed. Email: rebecca.@colorado.edu. chronous or diachronous, are unknown. Although much work This article contains supporting information online at https://www.pnas.org/lookup/suppl/ has focused on Cambrian sedimentary records that locally rest on doi:10.1073/pnas.1913131117/-/DCSupplemental. older strata or that unconformably overlie basement (7–9), First published April 27, 2020.

10172–10180 | PNAS | May 12, 2020 | vol. 117 | no. 19 www.pnas.org/cgi/doi/10.1073/pnas.1913131117 Downloaded by guest on September 24, 2021 Routt and Berthoud plutonic suites (29). The batholith is dom- inated by the coarse-grained Pikes Peak granite (30) that was emplaced at 1,066 ± 10 Ma (31). The batholith likely crystallized at depths <5 km based on the low water content and late- crystallization temperatures of 720 °C to 700 °C estimated for the magmas and may even have breached the surface (32). Three areas within the batholith characterized by concentric flow structures may underpin former calderas, further supporting shallow batholith emplacement (29). At Pikes Peak, one of these intrusive centers encompasses the southern lobe of the batholith (29). Initial rapid cooling following Pikes Peak batholith em- placement is documented by 10 hornblende 40Ar/39Ar dates from Fig. 1. Hypothesized exhumation histories below the Great Unconformity. 1,080 ± 3 to 1,068 ± 2 Ma, 4 biotite 40Ar/39Ar plateau dates from Orange bars depict geological constraints from Colorado. Hypothesis 1 (H1) 1,079 ± 3 to 1,073 ± 2 Ma, and K- 40Ar/39Ar data that depicts major erosion of the continents associated with assembly of the ∼ supercontinent Rodinia and mantle upwelling below it (14, 17). Hypothesis 2 suggest low-temperature cooling by 1,000 Ma (33). An apatite (H2) depicts major erosion associated with the early breakup of Rodinia (10–13, 15, 16). Hypothesis 3 (H3) depicts an association with the Cryogenian Snowball Earth glaciations (18–22). Hypothesis 4 (H4) depicts major erosion associated with the Cambrian transgressive buzzsaw and subsequent burial (4, 5), later rifting of Laurentian margins (23, 24), or the Pan-African and Transantarctic orogenies and construction of the supercontinent Pannotia (25, 26). An alternative hypothesis is that there are multiple Great Unconfor- mities representing diachronous regional phenomena, and these features de- veloped predominantly on and its conjugate rifted margins.

the Great Unconformity at specific localities by constraining Precambrian basement exhumation histories prior to overlap assemblages is required to test the competing hypotheses EARTH, ATMOSPHERIC, AND PLANETARY SCIENCES for Great Unconformity formation and tease out potential complexities in its origins. Recent work has used advances in thermochronology to con- strain major cooling events preceding Great Unconformity de- velopment in the Ozark Mountains of the central United States (16) as well as the Bighorn Mountains and Wind River Range in Wyoming (28). These studies inferred major erosion associated broadly with Rodinia supercontinent breakup (16, 28). However, the Cambrian sedimentary sequences above the Great Un- conformity at these localities only strictly limit development of the erosion surface to be Cambrian or older such that the ther- mochronologic data allow major erosion even into the latest Neoproterozoic. Consequently, these studies are unable to dis- tinguish among the hypotheses in Fig. 1. Integration of tighter Neoproterozoic and Cambrian geologic constraints with ther- mochronologic data is vital to resolve cooling and erosion his- tories at the level required to differentiate between models for Great Unconformity development. Here, we present an exceptional suite of Neoproterozoic and Cambrian surface constraints from the Late Pikes Peak batholith of the southern Colorado that document that the basement was at the surface by 676 ± 26 Ma and likely before 717 Ma. These relationships provide evidence that most erosion below the Great Unconformity occurred here prior to the Cryogenian Snowball Earth glaciations, thus en- abling discrimination among Great Unconformity hypotheses at this locality. Zircon (U-Th)/He (ZHe) data from samples im- mediately below the Great Unconformity are consistent with the geologic observations. We conclude by comparing these out- comes with those of recent work and consider the broader sig- nificance for a synchronous vs. diachronous origin of the Great Fig. 2. Geologic map of the Pikes Peak study area with sample locations Unconformities. marked. The Tavakaiv injectites that indicate the position of the Neo- paleosurface are also shown. SI Appendix, Fig. S1 is a regional Geologic Setting of the Pikes Peak Batholith and Great geologic map that shows the broader distribution of Tavakaiv exposures ∼ 2 Unconformity in Colorado across an 24,000-km region of the Colorado Front Range and nearby basement uplifts. Inset is a simplified map of the western United States that The 1.07-Ga Pikes Peak batholith is located in the southern shows the location of the field area and the distribution of Neoproterozoic Colorado Front Range (Fig. 2), where it intrudes ∼1.7-Ga strata across the western United States. DV, Death Valley; GC, ; metasedimentary rocks as well as ∼1.7- and ∼1.4-Ga rocks of the UM, Uinta Mountains; LIP, large igneous province.

Flowers et al. PNAS | May 12, 2020 | vol. 117 | no. 19 | 10173 Downloaded by guest on September 24, 2021 fission track (AFT) date of 447 ± 57 Ma is available from a Constraints on the Great Unconformity in Colorado from the sample near the top of Pikes Peak and attributed to resetting Tavakaiv Injectites during Phanerozoic burial heating (34, 35), but otherwise, the The Tavakaiv sandstone is an areally extensive clastic system of Pikes Peak granite is largely devoid of apatite and therefore, centimeter- to meter-scale dikes and sills that cuts Proterozoic lacks AFT (34) and apatite (U-Th)/He data. basement over an ∼24,000-km2 region of the Colorado Front The Great Unconformity in the study area is defined where Range and nearby basement uplifts (SI Appendix, Fig. S1) (42, Cambrian sandstone of the nonconformably 43). The sandstone consists of an indurated, poorly sorted, sits on Pikes Peak granite basement rock (Fig. 3A). Post-Sawatch subrounded to rounded, coarse-grained sand matrix with strata provide a record of subsequent episodes of exposure, subordinate, dispersed gravel to pebble-sized clasts of rounded burial, and reexposure of the Pikes Peak batholith. In some lo- quartz and angular granite and is generally maroon in color cations, arkosic sandstone of the Fountain For- owing to presence of authigenic Fe-oxide (44). Basement-hosted mation both overlies the Pikes Peak granite (Fig. 2) and contains are referred to as injectites because they require fluid clasts of the granite (36, 37). Latest Arapahoe For- overpressure for emplacement (42). The Tavakaiv injectites were mation sandstones also contain Pikes Peak granite clasts (38). formed by downward and lateral injection of liquified clastic sediment from a source at the surface (42). Multiple generations Maximum preserved thicknesses of Early Paleozoic sedimentary with clear cross-cutting relationships intruded the Pikes Peak rocks in the study region are <100 m, whereas those for the granite along a >75-km length of the Ute Pass fault system Pennsylvanian through Cretaceous section are ∼3.1 to 4 km – (Fig. 2). Tavakaiv dikes and sills also cut larger Tavakaiv sand- (39 41). Together, this indicates that the Pikes Peak granite was stone bodies within fault-bounded kilometer-scale “panels” be- at the surface in the Cambrian, was buried and reexposed in the tween strands of the Ute Pass fault (42). Pennsylvanian, and then, was reburied and again reexposed in Outcrop relationships indicate that most Tavakaiv injectites the latest Cretaceous. were emplaced in the near subsurface (42). Some bodies have The objective of our study is to determine when the Pikes Peak bulbous, curving margins that deform an intense fracture fabric batholith was first exhumed to the surface before Cambrian in granite host rock, suggesting that the host rock was intensely sandstone deposition in order to test the hypotheses for sub- weathered, malleable, and proximal to a free surface (Fig. 3B). Great Unconformity erosion. As described below, Tavakaiv Highly weathered, angular fragments of chloritic, oxidized gran- sandstone dikes and sills provide key constraints on the mini- ite commonly occur as clasts within strongly indurated, otherwise mum timing of initial granite exposure. pristine, unweathered injectites (Fig. 3C and SI Appendix, Figs.

A B

quartz sand injectite

Pikes Peak granite

C D

quartz sand injectite weathered granite chloritic fragments injectite Pikes Peak granite quartz sand injectite

Fig. 3. Photographs illustrating key Tavakaiv injectite, Sawatch sandstone, and Great Unconformity field relationships that constrain the position of the Neoproterozoic and Cambrian paleosurfaces. (A) Great Unconformity near Manitou Springs, Colorado. Pikes Peak basement with ∼1.5-m-diameter core stones (marked with the yellow arrow) onlapped by sandstone of the Sawatch Formation. The yellow star is where sample F1936 was collected ∼10 m below the Great Unconformity. (B) Bulbous Tavakaiv injectite, outlined in white, cutting weathered Pikes Peak granite near Buffalo Creek, Colorado. (C) Detail of the interior to a Tavakaiv dike near Buffalo Creek, Colorado. Injected sediment consists of mature quartz sand, with red color imparted by interstitial he- matite. Greenish-colored sand injectite contains abundant clasts of weathered granite, chlorite, and clays entrained from the weathered wall rock.The intermixed sediments display both sharp and gradational contacts. Image credit: Alec Lee (Colorado College, Colorado Springs, CO). SI Appendix, Fig. S2 shows the field context within a 7-m-wide sedimentary dike. (D) Great Unconformity at Perry Park with sandstone of the Sawatch Formation overlying Pikes Peak granite. The yellow star is where sample F1937 was collected ∼2 m below the Great Unconformity, which is marked by a white line.

10174 | www.pnas.org/cgi/doi/10.1073/pnas.1913131117 Flowers et al. Downloaded by guest on September 24, 2021 S2 and S3 A and B) (42). The degree of of the surface, have long lateral continuity along faults and fractures, engulfed fragments and flakes of granitic wall rock indicates lack quartz pebbles and weathered basement clasts, and likely prolonged residence of Pikes Peak granite at the surface that reflect emplacement depths of up to ∼1 km (42). caused formation of a deeply weathered granitic regolith prior to A Cryogenian age for the injectites is indicated by (U-Th)/He sand injection, entrainment of oxidized regolith fragments, and data on hydrothermal specular hematite (44). The specular he- cementation. Some centimeter-scale Tavakaiv dikes appear matite occurs in quartz–hematite veins inferred to have miner- controlled by brittle fracture geometries within the host granite, alized from 230 °C to 210 °C hydrothermal fluids during sand consistent with emplacement into fractured, weathered granite injection (44). The quartz–hematite veins cut and are cut by the near the surface (SI Appendix, Fig. S3C). Elsewhere, there is Tavakaiv dikes, suggesting coeval formation (44). Thirty-one clear evidence that some Tavakaiv dikes were injected more single hematite crystals of varying size isolated from poly- deeply. These dikes occur within competent granite bedrock in crystalline aggregates of two specular hematite vein samples incised canyons >300 m below the present-day regional land yielded (U-Th)/He dates of 717 ± 12 to 437 ± 7 Ma (44). The

1000 A PP1 PP2 PP3 800 F1936 F1937

600

400

ZHe date (Ma) 200

0 EARTH, ATMOSPHERIC, AND PLANETARY SCIENCES 0 500 1000 1500 2000 eU (ppm)

B Weathered Pikes Peak Ancestral Rockies granite clasts in Tava Paleozoic burial burial and injectites: Minimum age for and erosion GU erosion surface formation Laramide erosion Pikes Peak granite Cambrian Sawatch Pennsylvanian crystallization Great Formation overlies contains Pikes Peak (zircon U-Pb) Unconformity basement overlies basement granite clasts 0

2I

40

80 Sturtian Marinoan 120 Best-fit tT path Good-fit tT path Temperature (°C) 160 Acceptable-fit tT path Geologic constraint Exploration field 200 1000 800 600 400 0200 Time (Ma)

Fig. 4. ZHe data and thermal history modeling results for Pikes Peak granite samples. (A) Plot of ZHe date vs. eU, where the dashed curve marks the date–eU distribution predicted by the best-fit path in B. Uncertainties on individual ZHe dates are reported at 2σ and are based on the propagated analytical un- certainties on the U, Th, and He measurements. Uncertainties on eU values are conservatively assigned to be 15%. (B) Temperature–time diagram showing the inverse thermal history model result for the Pikes Peak ZHe data. This plot shows only the <200 °C temperature range, but the models begin with Pikes Peak granite emplacement at temperatures of 600 °C at 1066 ± 10 Ma followed by rapid cooling. Complete details regarding model inputs and applied geological constraints are in SI Appendix, Table S2. Although the best-fit path is shown to demonstrate that it reproduces the data in A, favoring the best-fit over the other “good-fit” paths is not statistically defensible, and we base our interpretations on the full range of good-fit results.

Flowers et al. PNAS | May 12, 2020 | vol. 117 | no. 19 | 10175 Downloaded by guest on September 24, 2021 dates show a broad positive correlation with crystal thickness, evidence for initial granite exposure and development of the Great consistent with He retentivity that varies with crystal size (44). Unconformity erosion surface by 717 Ma. The dates for the nine largest crystals were used to obtain a mean and uncertainty of 676 ± 26 Ma, favored as the timing of sand Constraints on the Great Unconformity in Colorado from injection, but this date can alternatively and more conservatively Zircon (U-Th)/He Thermochronology be considered a minimum age for Tavakaiv emplacement (44). Although the geologic information alone requires Pikes Peak Thermal history modeling using hematite He diffusion kinetic basement exposure prior to Cryogenian Tavakaiv sediment ac- data implies He loss from the hematite during subsequent burial cumulation and sand injection, we additionally acquired (U-Th)/ events (44) such that the oldest suite of four hematite dates that He thermochronology data for 30 individual zircon crystals from vary from 717 to 695 Ma could represent emplacement with the five Pikes Peak granite samples proximal to the Neoproterozoic remaining data array reduced to younger dates from variable and Cambrian paleosurfaces in an effort to refine the timing of He loss. initial basement exhumation (Materials and Methods). Rocks Detrital zircon U-Pb data from samples of the different cool as they are exhumed to the surface, and ZHe data can re- Tavakaiv generations across the region are consistent with the cord key portions of this cooling history. Three of our samples Cryogenian injectite age. Diagnostic zircon populations are (PP1 to PP3) were collected on the eastern side of Pikes Peak on similar to those of other Neoproterozoic siliciclastic strata across the footwall (western side) of the Ute Pass fault (Fig. 2). The the southwestern United States, suggesting approximately coeval closest identified Tavakaiv injectite exposures are in the footwall, ∼ formation from a common clastic Neoproterozoic source (43, 800 m east of the easternmost of these samples (PP1). Two of 44). The Tavakaiv detrital zircon data are also statistically dis- our samples are located immediately below the Great Un- tinct from detrital zircon age distributions in Paleozoic sand- conformity where it is overlain by the Cambrian Sawatch sand- stones (43, 44). In addition, a single detrital zircon from one of stone (Fig. 2). One of these samples (F1936) was collected ∼ the Tavakaiv samples yielded a date of ∼760 Ma. Although this 10 m below the sandstone where the Sawatch overlies core A result is insufficient to impose a firm maximum emplacement age stones in the Pikes Peak basement (Fig. 3 ) on the downthrown (eastern side) of the Ute Pass fault, ∼800 m northeast of sample without being reproduced (44), it is compatible with the Cry- ∼ ogenian hematite (U-Th)/He age for Tavakaiv injection. PP1. The other sample (F1937) was collected 2 m below the Great Unconformity (Fig. 3D) where the Sawatch overlies The Cryogenian Tavakaiv age and the geologic relationships ∼ documenting shallow injectite emplacement require that the Tavakaiv-injected Pikes Peak basement, located 43 km north of 1.07-Ga Pikes Peak basement underlying the Great Un- F1936 in Perry Park. Zircon data from all samples display similar < negative correlations between ZHe date and effective uranium conformity was exhumed from 5-km depth (29, 32) to the ± ± ∼ 2 concentration (eU), with dates ranging from 975 41 to 46 1 surface by Cryogenian time. The 24,000-km distribution of A SI Ap- identified injectites marks the position of a regional Cryogenian Ma over an eU span of 37 to 1,955 ppm (Fig. 4 and pendix, Table S1). This pattern is consistent with lower He re- surface that encompassed a broad swath of the paleolandscape tentivity at higher radiation damage dose (Materials and Methods). (SI Appendix, Fig. S1). The injectites temporally overlap with the The dates do not vary with grain size (SI Appendix,Fig.S4). 717- to 660-Ma Sturtian glaciation (15, 45, 46), which inspired Intracrystalline eU zonation likely contributes to ZHe data dis- the hypothesis that kilometer-scale ice sheets provided the con- persion beyond what can be accounted for by variability in bulk eU fining pressure for sand injection (44). Injectite emplacement and crystal size (47–49). was likely promoted by seismicity associated with basement We carried out tT (time–temperature) simulations of the Pikes fracturing and faulting during regional Neoproterozoic extension Peak ZHe data using the HeFTy program (50) and a zircon ra- associated with Rodinia supercontinent breakup (44) as man- SI Appendix diation damage accumulation and annealing model (ZRDAAM) ifested by injection of dikes up to 7 m in width ( , Fig. (51) to test for compatibility of the ZHe data with the geologic S2) along the extensional Ute Pass fault. Still earlier exposure of evidence for pre–717-Ma basement exhumation. Full model de- the granite to surface weathering is consistent with fragments of tails are in Materials and Methods and SI Appendix,TableS2(52). weathered Pikes Peak granite entrained in unweathered injec- B “ ” C SI Appendix A B Model results are in Fig. 4 .The best-fit tT trajectory repro- tites (Fig. 3 and , Fig. S3 and ). The cold duces the observed ZHe date–eU pattern (Fig. 4A,dashedcurve). conditions that prevailed during the 717- to 660-Ma Sturtian The key result is that, following shallow batholith emplacement glaciations were not conducive to regolith development. If the and initial rapid cooling, the suite of viable tT paths allows cooling > injectites formed during the 639- to 635-Ma Marinoan glacia- and exhumation to surface conditions anytime between ∼1000 and ∼ tion, a regolith could have formed between 660 and 640 Ma, 717 Ma. This outcome is thus consistent with the geologic evi- but a 640- to 635-Ma age of the injectites seems inconsistent with dence for pre–717-Ma basement exhumation and development of the existing geochronology. This suggests a minimum age of 717 the Great Unconformity erosion surface. Ma for weathering of the granite and therefore, for basement exhumation to the surface. Discussion In our study area, the Cambrian Sawatch sandstone overlies Development of the Great Unconformity Erosion Surface in Colorado Tavakaiv-injected Pikes granite, thus indicating little basement before Neoproterozoic Snowball Earth. Our geologic and thermo- exhumation between Cryogenian and Cambrian times. If sub- chronologic results provide evidence that the ∼1.07-Ga Pikes stantial exhumation had occurred, then the shallowly emplaced Peak batholith was exhumed and first exposed to weathering injectites would have been removed by erosion. Although in the between ∼1000 and 717 Ma, which led to development of the southern part of the batholith the Tavakaiv and Sawatch occur Great Unconformity paleosurface. This indicates that most ex- on opposite sides of the Ute Pass fault, farther north at Perry humation below the Great Unconformity in Colorado occurred Park the Sawatch sits directly on Tavakaiv-injected granite before the Sturtian glaciation and favors either Rodinia super- (Fig. 2). Locally, the basal Cambrian beds above the Great continent assembly before 850 Ma (Hypothesis 1) and/or Rodinia Unconformity drape over relief induced by variably resistant, breakup between 850 and 717 Ma (Hypothesis 2) as the cause(s) meter-scale core stones in an ∼100-m-thick weathered zone of (Fig. 1). Rodinia formed during the terminal Mesoproterozoic the Pikes Peak granite (Fig. 3A). This is clear evidence that a Grenville orogeny and rifted apart from ca. 850 to 700 Ma Pikes granite regolith was also present in the Cambrian. Al- (53–55). Grenville orogenesis produced Late Mesoproterozoic though the Pikes Peak batholith was later buried and reexhumed topography and magmatism across Laurentia from southeastern multiple times in the Phanerozoic, the Tavakaiv injectites provide Canada (56, 57) through the midcontinent (58) to the American

10176 | www.pnas.org/cgi/doi/10.1073/pnas.1913131117 Flowers et al. Downloaded by guest on September 24, 2021 Southwest (59). Mantle convection models suggest that a ring of Tavakaiv injectites were emplaced in both the immediate sub- subduction around the fully assembled Rodinia would have surface and up to depths of <1 km (42) such that their regional caused bipolar upwelling before 800 Ma (17), with the thick preservation and occurrence below the Sawatch indicates <1km supercontinent crust creating a thermal lid, inducing Late Mes- of cumulative Pikes granite erosion between Tavakaiv injection oproterozoic to Early intraplate magmatism throughout and Sawatch deposition. If the ∼100-m-thick weathered zone Rodinia (14, 60). In our study area, these processes could have with core stones under the Sawatch (Fig. 3A) is the same regolith induced Pikes Peak batholith magmatism, surface uplift, and intruded by the Tavakaiv injectites, then net denudation was as erosion (Hypothesis 1). Early Rodinia breakup along the Cor- little as <100 m from 717 to 510 Ma, which encompasses both dilleran margin associated with the Gunbarrel large igneous Cryogenian Snowball Earth events and Ediacaran glaciations. province (61) and Late Tonian basin formation (62, 63) also is a This direct evidence from our study area for limited de- feasible mechanism for exhumation. Neoproterozoic normal nudation during the Neoproterozoic glaciations is at odds with faulting along the Ute Pass fault (64) as well as detrital zircon recent work that indirectly inferred a global average of 3 to 5 km age distributions in Tavakaiv sandstone that resemble those of of erosion during this interval based primarily on preserved the Chuar–Uinta Mountains–Pahrump and Cryogenian basins sediment volumes in North America (i.e., Macrostrat), isotopic across the western United States are consistent with intra- proxy data (zircon eHf and O isotopes), and the impact cratering continental extension in the Pikes region in Cryogenian time (43, record (2). This proposed massive erosion event is inconsistent 44, 64), which may have exhumed the basement to the surface not only with our observational data but also with other geologic (Hypothesis 2). constraints on North America indicating far less than 3 to 5 km Other recent thermochronologic work also constrains Neo- of Cryogenian erosion. These include preservation of >1kmof proterozoic cooling and denudation events. K-feldspar 40Ar/39Ar ca. 720-Ma basalt in the Natkusiak Formation of northwest multidiffusion domain data from the southern Canadian shield Canada (66); >450 m of ca. 720-Ma basalt in the Kikiktak For- record significant post–1.0-Ga cooling attributed to exhumation mation of Arctic Alaska (67); and 780- to 717-Ma sedimentary associated with magmatic underplating during Midcontinent Rift basins in Grand Canyon, in Death Valley, in the Uinta Moun- extension and Grenville orogenesis (56). ZHe data from the tains of Utah, throughout the Canadian Cordillera (63), and in Ozark Plateau of the US midcontinent (16) as well as data from the central Appalachians (68). Moreover, sedimentation rates on uplifts in the Wyoming (28) were used to decipher Neo- the continental margins were anomalously low during the Cry- proterozoic cooling and erosion that overlaps with Rodinia su- ogenian (69), indicating reduced rather than increased erosion of percontinent breakup. Titanite and zircon (U-Th)/He data from the continents. Thus, the geologic observations and available

southern Africa indicate substantial ∼1.1-Ga burial of the thermochronologic data do not support a single immense, EARTH, ATMOSPHERIC, AND PLANETARY SCIENCES Kaapvaal craton during Rodinia assembly and imply Neo- worldwide denudation episode during the Neoproterozoic Snowball proterozoic cooling and erosion (65). However, none of these Earth. investigations require an age for the Great Unconformity erosion surface older than the Sturtian Snowball Earth as necessitated by Diachronous Tectonic Origin of Great Unconformities. In contrast to the geological relationships in the Pikes Peak region. Instead, the notion that Great Unconformity erosion was a synchronous Precambrian rocks in the Ozark Plateau and Wyoming, southern global phenomenon (2, 4, 5), we propose that it was diachronous Canadian shield, and Kaapvaal craton are overlain by Cambrian, and in places, composite, such that multiple Great Unconfor- upper , or units, respectively. These mities were generated by more than one denudational episode relationships and the thermochronologic data impose Paleozoic with tectonic causes that varied geographically. During the minimum ages on erosion surface development at these locations Neoproterozoic, different continental margins were variably af- in contrast to the 717-Ma minimum age bound in our study. fected by collisional tectonism and rifting during Rodinia and Consequently, our dataset is the only one that allows for dis- Pannotia/Gondwana assembly and breakup. For example, on the crimination among any of the hypotheses in Fig. 1. North American margins, there is not one Great Unconformity As an example, we carried out thermal history simulations of characterized by a single set of relationships, but rather there is a the published ZHe data from the Ozark Mountains of the central composite surface with overlapping unconformities that en- United States (16) to compare how well the timing of Great compasses multiple stages of Rodinia and Pannotia rifting (27, Unconformity erosion surface formation in the Ozarks is re- 63, 70–72). The Appalachian margin was affected by the exten- solved relative to our Pikes Peak study region. We applied the sive Grenvillian orogenic system while the Cordilleran margin same geologic constraints as in ref. 16, with full model details in was not; the former saw Ediacaran magmatism and rifting while Materials and Methods and SI Appendix, Table S3. We find that the latter underwent extension and rift-related volcanism pri- the good-fit tT paths for the Ozark data allow basement exhu- marily in the Tonian to Cryogenian. On other continents, such as mation to the surface anytime between 957 and 511 Ma, with the Kalahari craton and across much of Africa, widespread un- major cooling to <100 °C permitted as early as 1037 Ma and as conformities are overlain by Late Neoproterozoic and Cambrian late as 600 Ma (SI Appendix,Fig.S5A and B). This outcome is strata that formed in foreland basins associated with the Pan- far less restrictive than our Pikes Peak result, which is due to African orogeny (73). Thus, we suggest that sub-Great Un- Cambrian rather than Cryogenian geologic relationships im- conformity erosion occurred at different times and for different posing the minimum age on Great Unconformity development reasons in different places, generating multiple Great Uncon- in the Ozarks (SI Appendix,Fig.S5B and C). This highlights formities. Our results resolve a pre-Sturtian Snowball Earth age the value of paleosurface observations for determining the for the Great Unconformity erosion surface in one region of Great Unconformity’s age at the level required to differenti- North America. Additional direct constraints on the age, dura- ate among the hypotheses for its origin as enabled by the tion, and spatial extent of Great Unconformity formative events Cryogenian Tavakaiv injectite relationships in the Pikes Peak elsewhere are required to develop a global understanding of the region. Great Unconformities and their significance.

Limited Erosion during Neoproterozoic Snowball Earth. Our dataset Conclusions from Pikes Peak indicates that most erosion below the Great Our study documents that the majority of erosion below the Unconformity occurred before the onset of the Sturtian Snow- Great Unconformity at Pikes Peak predates the Sturtian-age ball Earth here, with limited basement denudation between 717 Tavakaiv injectites. This result is inconsistent with recent pro- Ma and deposition of the Sawatch sandstone at 510 Ma. The posals that the Great Unconformity formed in a single global

Flowers et al. PNAS | May 12, 2020 | vol. 117 | no. 19 | 10177 Downloaded by guest on September 24, 2021 exhumation pulse during the Neoproterozoic Snowball Earth (2) Thermal History Modeling of the Zircon (U-Th)/He Data. We simulated the Pikes and is responsible for a putative transition to modern tectonics Peak ZHe data using the HeFTy program (50) and the ZRDAAM (51). The (3). Rather, we propose that there are multiple, regionally dia- imposed thermal history constraints (thick green boxes in Fig. 4B) include 1) shallow emplacement of the Pikes Peak batholith at 1.07 Ga (31) followed by chronous Great Unconformities with tectonic origins. Although 40 39 rapid cooling based on hornblende, biotite, and K-feldspar Ar/ Ar data the relationships suggest that Neoproterozoic glacial erosion did (33); 2) attainment of surface conditions prior to the Sturtian glaciation at not singularly generate the Great Unconformity, it is possible 717 Ma, maximum temperatures of 30 °C during the Snowball glaciations that Rodinia rifting increased global weatherability and created based on equatorial ice sheet thicknesses of 0 to 3 km in Snowball Earth the tectonic context for the initiation of Snowball Earth (11, 15, climate models (87) and the position of Colorado at or near the equator 16). If the Great Unconformities are a worldwide phenomenon, during this time (88), and surface conditions by the onset of 510-Ma Sawatch it must relate to the Tonian removal of topography generated in sandstone deposition above the Great Unconformity (24); 3) early Paleozoic the assembly of Rodinia, with erosion and sedimentation histo- burial heating followed by surface conditions by the time of deposition of the Pennsylvanian Fountain Formation on the Pikes Peak granite; and 4) ries diverging during diachronous and prolonged Late Neo- post–300-Ma burial heating with surface conditions by 64 Ma based on the proterozoic supercontinent breakup and glaciation. occurrence of Pikes Peak granite clasts in the Arapahoe Formation (37). We applied a near-surface “exploration field” box between Pikes Peak granite Materials and Methods emplacement and surface conditions at 717 Ma that did not restrict the Zircon (U-Th)/He Thermochronology. (U-Th)/He thermochronology is based on models but ensured that this regime of tT space was fully tested. the radioactive decay of trace U and Th (and to a lesser extent, Sm) to 4He in The models simulate random tT paths forced through the geologically a mineral’s structure and on thermally controlled volume diffusion of the defined tT constraints but otherwise explore the full model tT space. The ZHe 4He atoms. At high temperatures, He escapes from the crystal, while at low data from all of the Pikes Peak granite samples were split into four eU–date temperatures, it is retained. The temperature range for He retention depends bins, with the mean values for the ZHe date, eU, and equivalent spherical on the mineral’s He diffusion kinetics, which is influenced by crystal lattice radius of each bin used as model inputs. Paths are considered “good” or structure, grain size (74), and radiation damage (51, 75–77). Parent–isotope “acceptable” if they reproduce the input data to specified statistical zonation (48–50), He injection from neighboring phases (78), and grain frag- thresholds (SI Appendix, Table S2) (50, 89). The results indicate cooling to mentation (79) are additional factors that can influence (U-Th)/He data. surface temperatures between ∼1000 and 717 Ma, demonstrating that the In zircon, radiation damage exerts strong control on He retentivity and the ZHe data are consistent with basement exhumation to the surface before – associated data patterns. Damage accumulation initially causes the He closure 717 Ma. We emphasize that the key conclusion of pre 717-Ma development of the sub-Great Unconformity erosion surface is derived from the Cry- temperature (Tc) to increase from ∼140 °C to 220 °C, but after a damage ogenian Tavakaiv injectite relationships, and thus, it is independent of the percolation threshold is surpassed, the Tc progressively decreases to <50 °C (51, 80). Heating can anneal radiation damage, which in turn, affects He ZHe data and any shortcomings of the ZRDAAM. retentivity (76, 80). For zircon suites with a shared thermal history and a span We also simulated the published ZHe data (16) from the Ozark Mountains of U-Th, radiation damage can generate positive or negative correlations of the central United States to evaluate how well those results resolve the timing of erosion below the Great Unconformity compared with our data between ZHe date and eU (eU = U + 0.235 × Th) (51). These patterns develop from Pikes Peak (SI Appendix, Fig. S5 A and B). We again used inverse if the tT history is characterized by enough time for damage accumulation modeling with the HeFTy program (50) and the ZRDAAM (51). We applied and development of variable He retentivities in a mineral suite followed by the same geologic constraints as in ref. 16 (full details are in SI Appendix, reheating, partial resetting, and/or protracted cooling through the mineral’s Table S3), which include 1) granite crystallization at 1.47 to 1.38 Ga at partial retention zone (81). However, even if the mineral suite develops temperatures of 600 °C to 550 °C followed by cooling to the surface by 1.35 variable He retentivities, tT paths with only rapid cooling and/or reheating Ma based on volcanics of this age overlying the basement, 2) allowing burial and complete resetting will not induce date–eU correlations (81). reheating between 1350 and 510 Ma to temperatures sufficient to reset the A zircon radiation damage accumulation and annealing model (ZRDAAM) ZHe data followed by surface conditions by the onset of 509-Ma Lamotte (51) is available that allows for quantitative thermal history interpretation of sandstone deposition above the Great Unconformity, 3) Paleozoic burial ZHe datasets. The high-damage end of the ZRDAAM is least well constrained reheating to account for deposition of Paleozoic sedimentary rocks in the (51, 82, 83). Recent work shows that bulk radiation damage annealing in region, 4) temperatures <120 °C after 209 Ma based on AFT dates of 209 to zircon requires hotter temperatures for longer intervals than assumed by the 185 Ma, and 5) surface conditions today. We imposed a constraint box for fission track annealing model currently used by the ZRDAAM (84). Active the AFT data because the full AFT date and track-length data were not research is underway by different groups to improve understanding of zir- published, and therefore we are unable to model them. We applied a near- con He diffusion and radiation damage annealing. surface exploration field for the 500-My preceding surface conditions at 509 Zircon crystals were isolated from the samples using standard crushing, Ma that did not restrict the models but ensured that this regime of tT space water table, heavy-liquid, and magnetic separation techniques. Individual was fully tested. It seems that this exploration box was not imposed in the crystals were handpicked for analysis based on crystal shape, size, and color models reported in ref. 16. We suspect that the omission of this box caused using a Leica M164 binocular microscope equipped with both polarized the models in ref. 16 to not fully explore this key region of tT space and to transmitted and reflected light. Zircons were specifically selected to have a yield results that artificially suggest a more restricted interval of pre-Great range of opacity and discoloration, with the goal of analyzing crystals that Unconformity erosional cooling than constrained by the data in reality. encompass a broad range of radiation damage (85). Grains were photo- The ZHe data from all four samples were split into five eU–date bins, with the graphed, measured, packed into HCl-cleaned Nb packets, and analyzed for mean values for the ZHe date, eU, and equivalent spherical radius of each bin He using an ASI Alphachron He extraction and measurement line in the used as model inputs. We additionally included the apatite (U-Th)/He data from University of Colorado Boulder Thermochronology Research and In- sample 14OZ11 in this simulation and used the apatite radiation damage and strumentation Laboratory (CU TRaIL). Degassed crystals were then retrieved, accumulation model for He diffusion kinetics (81). We simulated this sample spiked with a calibrated 235U-230Th-145Nd tracer, and dissolved using pres- because the apatite (U-Th)/He data are reproducible, unlike the dispersed re- surized, high-temperature HF-HCl acid–vapor dissolution vessels. Dissolved sults for sample 14OZ12. SI Appendix,Fig.S5B shows the results of this thermal samples were measured for U, Th, and Sm on either an Agilent 7900 history simulation. The important outcome is that the data allow basement quadrupole inductively coupled plasma-mass spectrometer (ICP-MS) in the exhumation to the surface at any time between 957 and 511 Ma prior to de- CU TRaIL or a Thermo-Finnigan Element2 sector field ICP-MS. position of Cambrian sandstone above the Great Unconformity. This result is far The 4He atoms travel up to 20 μm during α-decay, requiring that a cor- less restrictive than that in our Pikes Peak study, where the observations and rection be applied based on crystal size and geometry to account for He lost data require that the basement below the Great Unconformity was exhumed ∼ owing to this effect. Standard α-ejection corrections were made using the to the surface between 1000 and 717 Ma (SI Appendix,Fig.S5B and C). approach of ref. 86. Crystal volumes used to compute isotopic concentra- tions were also calculated using the idealized crystal morphologies of ref. 86. ACKNOWLEDGMENTS. This work was supported by NSF Division of Earth Sciences Grants 1759200 and 1916698 (to R.M.F. and F.A.M.). NSF Division of We assign a conservative 15% uncertainty to eU values to account for ob- Earth Sciences Grants 1126991 and 1559306 (to R.M.F.) provided support for served variations from ideal geometries using the approach of ref. 77. Un- the CU TRaIL instrumentation used to acquire the ZHe data. We thank Jim certainties on individual ZHe dates are reported at 2σ and are based on the Metcalf for assistance obtaining the (U-Th)/He results. We appreciate the propagated analytical uncertainties on the U, Th, and He measurements. helpful reviews provided by two anonymous reviewers that improved the All data are available in SI Appendix, Table S1. clarity of the manuscript.

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